With modern magnetic recording media, there is a constant desire for higher storage capacities. faster access times and higher transmission rates of the stored information. For magnetic recording media, these requirements mean on the one hand a continuous increase in the relative speed between medium and magnetic head and on the other hand a continuous reduction in the thickness of the magnetic layer. For example, magnetic recording media having a high storage capacity now have magnetic layers which are less than about 1 .mu.m thick and relative speeds between recording medium and head which are in the region of several meters per second.
Particularly in the case of data diskettes with storage capacities of 100 megabytes or more, the thickness of the recording layer is already substantially below 0.5 .mu.m with relative speeds of more than 10 m/s. In this type of magnetic media, the magnetic head frequently remains on one and the same track during use, setting extreme requirements with respect to the abrasion resistance of the recording layer.
To meet this requirement, magnetic recording media in which a binder-free ferromagnetic metal layer was applied in a very small thickness by means of a vacuum technique have been developed in recent years. Although these metal evaporated recording media achieve a very high playback level, mass production of such media still presents considerable difficulties in comparison with magnetic recording media in which the magnetic pigments are dispersed in binders. Moreover, these ME tapes change under the influence of atmospheric oxygen. However, it has recently been possible to meet the requirement for small layer thickness also by means of a thin magnetic layer in which the finely divided magnetic particles are dispersed in a polymeric binder matrix and in which this layer is cast onto a nonmagnetic lower layer. Such recording media are described, for example, in U.S. Pat. No. 2 819 186, DE-A 43 02 516, EP 0 520 155, EP 0 566 100, EP 0 566 378, EP 0 682 802 and DE-A 44 43 896, 195 04 930, 195 11 872, 195 11 873, 195 11 875 and 195 11 876.
The abovementioned magnetic recording media describe media which have a two-layer structure and in which the upper, magnetic layer has a thickness of from 0.01 to slightly below 1 .mu.m, preferably 0.1-0.4 .mu.m. The thickness of the lower, nonmagnetic layer is 0.5-8 .mu.m. The upper layer preferably contains finely divided magnetic metal or metal alloy particles while the lower, nonmagnetic layer contains finely divided nonmagnetic pigments which in some cases have an acicular structure, as described, for example, in the abovementioned publications 20 EP 0 566 378 and EP 0 682 802. From a comparison of the thicknesses of the part-layers, it is clear that the mechanical properties are essentially determined by the thicker lower layer.
In conventional recording media, the mechanical stability and the abrasion resistance of magnetic recording media can be increased by adding acicular pigments in contrast to pigments of isotropic shape, particularly when the pigments are arranged in a suitable manner. In the case of media which have a preferred direction, such as, for example, tape-like magnetic recording media, an arrangement in this direction is advisable, as described in the PCT application 96/30900 of the same Applicant. In the case of acicular magnetic pigments, this orientation can be further substantially improved by orienting the pigments in a strong magnetic field.
To ensure that the required high recording density is achieved, acicular pigment particles can be used only in the nonmagnetic lower layer for improving the mechanical properties. In this application, the orientation of the nonmagnetic acicular pigment particles is supported by the coating process itself since, owing to the shear gradient inevitably produced during application, some of the pigment needles are oriented in the coating direction.
However, in the case of disk-shaped recording media, i.e. floppy disks, an anisotropic orientation is harmful since the relative direction of movement of the magnetic head relative to the preferred direction of the pigment needles changes continuously during one revolution. Here on the contrary it is important as far as possible to ensure no anisotropic properties of the recording medium, i.e. neither mechanical nor magnetic.
It is an object of the present invention to provide a magnetic recording medium of the generic type stated at the outset which, in the ready-to-use state, has neither magnetic nor mechanical anisotropy and at the same time has improved abrasion resistance and good mechanical stability.